1. Field of the Invention
The present invention relates to an active material for a rechargeable lithium battery and a rechargeable lithium battery including the same. More particularly, the present invention relates to an active material for a rechargeable lithium battery having excellent cycle-life characteristics and high electric conductivity and a rechargeable lithium battery including the same.
2. Description of the Related Art
Lithium rechargeable batteries have recently drawn attention as a power source of small portable electronic devices. The rechargeable lithium batteries use an organic electrolyte solution and thereby having twice the discharge voltage of a contemporary battery using an alkali aqueous solution. Accordingly, the rechargeable lithium batteries have high energy density.
A battery unit of a conventional lithium secondary battery includes a plurality of positive and negative electrode plates, and a separator interposed between the electrode plates. The battery unit is wound in a jelly-roll type in which a positive electrode plate, a separator and a negative electrode plate are sequentially disposed.
Each of the positive and negative electrode plates includes an electrode current collector and an electrode active material layer coated on the surface of the electrode current collector.
For positive active materials of a rechargeable lithium battery, lithium-transition element composite oxides being capable of intercalating lithium, such as LiCoO2, LiMn2O4, LiNi1-xCoxO2 (0<x<1), and so on, have been researched.
For negative active materials of a rechargeable lithium battery, various carbon-based materials such as artificial graphite, natural graphite, and hard carbon, all of which can intercalate and deintercalate lithium ions, have been used. Recently, non-carbon-based negative active materials have been actively researched.
Since the non-carbon-based negative active material, such as Si, has low conductivity, research on improving the conductivity of non-carbon-based negative active material has been actively undertaken. For example, a method for growing carbon nanotubes or carbon nanofibers on the surface of Si has been researched. The carbon nanotubes or carbon nanofibers have a problem, however, that the carbon nanotubes and carbon nanofibers have a very large specific surface area and a strong Van der Waals force, and thus the carbon nanotubes and carbon nanofibers tend to tangle easily. In addition, the carbon nanotubes or carbon nanofibers are made from a graphite mesh with well developed crystallinity, and thereby having a very strong hydrophobic property.
The carbon nanotubes and carbon nanofibers are very strongly hydrophobic, have a very small size of several to tens of nanometer (nm), and are grown on the surface of an active material. The carbon nanotubes and carbon nanofibers undesirably prevent a binder that is used for binding the active materials and for binding the active materials to a current collector, to penetrate between the carbon nanotubes and carbon nanofibers. Therefore, the adherence of the active material to a current collector substantially deteriorates. In addition, the active materials do not sufficiently interconnect with each other, and thus are present in a form of a mass. Therefore, a rechargeable lithium battery that is constructed with the active materials having carbon nanotubes or carbon nanofibers, does not have an excellent electrically conductive network.
Such a problem is very serious when an aqueous binder such as polyacrylic acid is used, since the active material on which carbon nanotubes or nanofibers is grown has a very strong hydrophobic surface.